The invention relates to a DC/DC up/down converter, comprising:
first and second input terminals;
first and second output terminal;
a coil;
first switching means operatively connected to provide a conduction path from said first input terminal to said coil;
second switching means operatively connected to provide a conduction path from said first switching means and said coil to said second input terminal;
third switching means operatively connected to provide a conduction path from said coil to said first output terminal;
fourth switching means operatively connected to provide a conduction path from said coil and said third switching means to said second output terminal;
control means operatively connected for controlling said switching means;
reference voltage means for providing a reference voltage; and
comparator means for providing a comparison signal for said control means in response to comparison of an output voltage at said output terminals with said reference voltage.
A DC/DC up/down converter of this type is known from International patent application WO 95/34121 of applicant.
This known switched DC/DC converter is a combination of a so-called boost and buck converter. In the boost mode, the first switching means are in a conductive state, i.e. closed, and the second switching means are non-conductive, i.e. open. The third and fourth switching means are operated for up-converting an input voltage applied at the input terminals.
In the buck mode, the third switching means are closed and the fourth switching means are open, such that with the first and second switching means the input voltage can be down-converted to an output voltage lower than the input voltage.
A xe2x80x9cdirectxe2x80x9d converter is realized by directly coupling the input voltage, applied at the input terminals, to the output terminals, by closing the first and third switching means and opening the second and fourth switching means.
For voltage regulation purposes, the known DC/DC converter individually switches between the three possible modi, i.e. the boost mode, the buck mode and the direct mode, following a reference voltage window determined by a first (high) and a second (low) reference voltage. If the output voltage in the direct converter mode becomes lower than the (low) second reference voltage, the converter is switched to its boost mode of operation. If the output voltage in the direct converter mode rises above the (high) first reference voltage, the converter is switched to the buck mode configuration. Due to the switched operation of the converter, at the output voltage a low frequency (LF) voltage ripple is induced.
In practice, it has been found that for a number of supply voltage sensitive applications, the LF voltage ripple caused by the reference voltage window, is not acceptable.
Accordingly, it is an object of the invention to provide an improved DC/DC up/down converter having a less output voltage ripple due to the switched control of the converter.
This is, according to the invention, achieved in that the control means are arranged for controlling the switching means in a three phase conversion cycle wherein
phase 1 comprising said first switching means and said fourth switching means in a conductive state and said second switching means and said third switching means in a non-conductive state;
phase 2 comprising said first switching means and said third switching means in a conductive state and said second switching means and said fourth switching means in a non-conductive state;
phase 3 comprising said second switching means and said third switching means in a conductive state and said first switching means and said fourth switching means in a non-conductive state.
Instead of applying the up- and down-conversion sections of the converter separately or individually, in the converter according to the invention a combined three phase control of the switching means is provided. With the three phase control according to the present invention, no voltage window is required for regulating the output voltage of the converter, thereby achieving a less disturbing output voltage ripple.
In a further embodiment of the invention, by adding a fourth phase wherein the switching means are all in a non-conductive state, the converter can be operated in a continuous or PWM (Pulse With Modulation) mode, wherein the current through the coil does not become zero and in a discontinuous or PFM (Pulse Frequency Modulation) mode wherein the current through the coil becomes zero. In the PWM mode the time duration of the fourth phase is zero whereas in the PFM mode the fourth phase introduces a wait period wherein the current through the coil becomes zero. Thus, a zero length wait cycle is associated with PWM and a non-zero length wait cycle is associated with PFM. Switching between PFM and PWM can be made automatically. The length of the wait cycle determines the type of mode.
In a preferred embodiment of the converter according to the invention, the first and second phase have a fixed duration, whereas the duration of the third phase is varied for output voltage regulation purposes.
In PFM or discontinues mode, the third phase ends at a predefined length or when the current through the coil becomes zero. Then, the control means waits until the output voltage drops below the reference level, to start a new conversion cycle with phase 1.
Preferably, switching means constructed as MOS (Metal Oxide Semiconductor) transistor means are used, having their control terminal (gate) connected with the control means for controlling the closed or open state of the transistors.
In a further embodiment of the invention, the second switching means are replaced by a first diode, arranged to provide a conductive path from the second input terminal to the first switching means and the coil, and the third switching means are replaced by a second diode arranged to provide a conductive path from the coil to the first output terminal.
The second and third switching means are optional, however if used, these switches increase the power conversion efficiency of the DC/DC converter by having none or a much less voltage drop in their conductive state compared to diodes. Those skilled in the art will appreciate that, in a practical embodiment, a parallel arrangement of the second switching means and the first diode as well as a parallel arrangement of the third switching means and the second diode is of advantage, for protection purposes of the second and third switching means, in particular if MOS transistors are used as switching means.
Preferably, the DC/DC up/down converter according to the invention comprises a smoothing capacitor connected across the output terminals.
The invention also relates to a power supply comprising a DC/DC up/down converter as disclosed above, and arranged for receiving an input voltage at the first and second input terminals and for providing an output voltage at the first and second output terminals.
The DC/DC converter according to the invention is of particular advantage if applied in a portable electronic appliance, such as, but not limited to, camera chips and the like.